This invention relates to a threaded load transferring attachment for a device having high strength, high elastic elongation, and high damping characteristics. More particularly, this invention relates to an apparatus and method for threaded attachment of a device made of shape-memory effect alloy, such as Nitinol, to another member for transferring loads between the device and the other member.
Shape memory effect alloys are intermetallic compounds that have some characteristics that would make them good candidates for many types of load transferring devices. Type 55 Nitinol, an intermetallic compound of approximately 55% nickel and 45% titanium, is one such alloy. In certain metallic states, its yield strength increases as work is applied, and it has a remarkable ability to absorb and dampen vibration. In certain metallic states, the alloy can undergo elongation of as much as 60% exerting an increasing resistance and elastic restoring force which would make it ideal for a self-locking, strain indicating fastener. When the alloy is strained in its Martensitic state and then heated to its Austenitic transition temperature, it spontaneously exerts a restoring force on the order of 100 KSI to restore the material to its pre-strained shape. This shape-memory effect of Nitinol makes its use in actuators particularly attractive because such actuators can be made with no moving parts such as electric motors, and without pyrotechnic gas generators or hydraulic systems.
Despite this potential, shape memory effect alloys have not been widely used in load transferring applications, primarily because of the difficulty in attaching the load transferring device to the load bearing member. Threading, the simplest and most widely used fastening technique for connecting a load transferring device to a load bearing member, has not been used for Nitinol because it is very difficult to cut, apparently because of its characteristic of increasing yield strength as cold work is applied. Even the hardest threading tools are quickly dulled or broken when attempting to cut Nitinol. Thread grinding of Nitinol would be slow and cause rapid wear of the grinding wheels, hence it would be uneconomical and unsuited to high volume production.
There are other techniques for connecting a Nitinol load transferring device to a load bearing member, but they are usually time consuming, inconvenient, expensive, not removable, and/or prone to failure. They include welding, clamping, crimping and separate fasteners. The use of fasteners is difficult because it usually requires drilling a hole in the Nitinol element, but there have been no known practical methods for production drilling of Nitinol; its increasing strength as cold work is applied quickly ruins ordinary drills. Clamping and crimping are difficult processes to control for consistent quality, and they tend to loosen over time because of vibration and thermal expansion. Welding produces a permanent connection which is often undesirable, and it creates a heat affected zone in the Nitinol that can change the desirable metallurgical characteristics of the material. These methods are used occasionally because there have been no known processes for threading Nitinol material. It would be a significant advance in the art to have available a fast, inexpensive and precision process for making threads in Nitinol and other shape memory effect alloy elements for making a fast, convenient and secure attachment for the element to a load.
When strained up to 8% in its Martensitic state and then heated to its transition temperature, Nitinol spontaneously exerts a restoring force equivalent to about 100 KSI to return to its pre-strained shape. This shape memory effect of Nitinol has been utilized to make Nitinol actuators, used for example to deploy missile fins after launch from a launch tube. Such an actuator includes a Nitinol element, such as a wire or ribbon, strained in its Martensitic state by as much as 8% and connected between the movable member (such as the missile fin) and a fixed member. A source of heat is provided for the Nitinol element to raise its temperature to the Austenitic transition temperature, whereupon it will exert a substantial force to return to its pre-strained shape. The source of heat can be a pyrotechnic or a resistance heating element surrounding the Nitinol element, or more typically, can be a source of electric power for passing a current through the Nitinol element itself, thereby raising its temperature by resistive heating.
A need exists for a blind-side capture device that is reliable, simple, light weight, inexpensive and remotely operable. One application for such a device is in spacecraft wherein a deployable structure, such as a pivoted arm or boom, must be secured permanently in its deployed position after it is deployed. Spacecraft and many other systems need reliable mechanisms, especially when the consequences of failure of the mechanism could be failure of the entire system. Reliability is often inversely proportional to complexity, so simplicity is a virtue in such systems, especially when it also saves weight and cost. The actuation of the latch in such fasteners is conventionally done by an electric motor or by a pyrotechnic device. Motors are heavy, expensive and failure prone. Pyrotechnics are usually fairly light weight, but produce undesirable shock and fumes that can be damaging to sensitive instruments, and the speed of actuation is difficult to control. If a blind side fastener could be actuated by a Nitinol actuator element instead of motors or pyrotechnics to secure a deployable structure in its deployed position, it would provide the needed capability and reliability without shock or fumes while reducing the cost and the weight of the mechanism to do the job.
Another actuator with many actual and potential uses in aerospace and other applications is the pin puller. A pin puller is a device having a pin supported at its two ends, releasably supporting a load on the middle section of the pin between the two supports. The load can be remotely released by axially withdrawing the pin from one of the supports and into the other support. Conventional pin pullers use pyrotechnics to pull the pin, but pyrotechnics have come into disfavor because of the risk to personnel installing the pyrotechnics, and also because of the shock and fumes produced when the pyrotechnic is initiated. However, they are used anyway because heretofore there have been no alternatives that matched the simplicity and reliability of the pyrotechnic pin puller. A pin puller that could use a Nitinol actuator element to withdraw the pin would provide the same or superior simplicity and reliability without the danger, shock, and fumes produced by pyrotechnic pin pullers.
Equipment and machinery mounts are widely used throughout industry and in consumer products to support machinery and equipment, and to isolate it from vibration, or isolate the structures on which they are mounted from vibration which the equipment or machinery produce. Motors and compressors are common examples of machinery that produces vibration, and this machinery is often mounted on vibration isolating mounts. The mount is often a resilient device, such as spring feet for mounting a compressor, and sometimes includes a damping device, combined sometimes in a single element such as an elastomeric pad. These devices usually perform adequately when they are new, but are subject to fatigue and deterioration with age and gradually lose their vibration isolating qualities as they age.
Nitinol functions well as a spring because of its high elastic elongation capability in the xe2x80x9csuperelasticxe2x80x9d form, and because, in both its Martensitic binary state and the superelastic form, it also has a damping capability that enables it to absorb a large percentage of the energy in vibrations. Moreover, it is virtually inert and unaffected by very high temperatures, so it can withstand environments that would quickly destroy an elastomeric mount. It can be easily tuned to provide the desired spring rate, and its damping characteristics enable it to optimally absorb characteristic vibrations from any particular piece of machinery. However, it has not been widely used as a machinery or equipment mount, in part because its panoply of characteristics have not been appreciated for what they can offer in a vibration isolation mount, even though the material has been available for many years. Moreover, Nitinol is difficult to work using conventional metal working techniques, and there have been no practical and economical methods of attaching the mount to the supporting and supported structures, so those skilled in the art have concentrated their efforts on easier materials to work with.
Conventional threaded fasteners are used ubiquitously in endless applications and usually perform adequately provided they are installed properly and are not subjected to stress or vibration that exceed their stress capabilities or their fatigue limits. However, those limits can be reduced by environmental influences, such as temperature or corrosive chemicals.
Beside environmental factors, the primary cause of failure of conventional fasteners is loosening under the influence of vibration and thermal cycling. Conventional fasteners also become loose if they were not properly tightened when they were initially installed to produce sufficient strain in the fastener to maintain pressure between the facing flanks of the threads on the nut and bolt when subjected to vibration. The torquing of fasteners is a difficult process to control because of the numerous variables that relate the applied torque to the tensile strain induced in the fastener. Since the strain cannot be conveniently measured directly, the applied torque is measured and is related to the strain induced in like fasteners under ideal conditions of lubrication, fit, finish, etc. When the actual conditions vary from the ideal conditions, the applied torque will not produce the desired strain in the fastener. Thus, the fastener art has long needed a fastener that is self-locking, that is, a fastener that is secure against loosening under the effects of vibration and thermal cycling, and also provides a direct indication of strain induced by torquing during fastener installation.
Numerous machines have stop pins for stopping the travel of a moving structure. Stop pins are in various forms, but often take the form of a screw or bolt head attached to a fixed structure in the path of a moving mechanism to halt the movement when the moving mechanism engages the stop pin. Sometimes the stop pin actually takes the form of a pin, threaded into the fixed structure at the desired location. Occasionally a stop pin is covered with a resilient material such as an elastomer or the like to help dissipate the impact, but such materials seldom survive for long except in very benign and low stress environments.
Conventional stop pins are afflicted with three intractable and related problems: shock, vibration and fatigue. When the stop pin takes the full impact of the moving mechanism, it transfers the momentum of the moving mechanism to the fixed structure. The resulting shock is transmitted through the moving mechanism and also through the fixed structure with possible long term injurious consequences, and the stress can accumulate quickly on the fatigue curve to cause early failure of the pin, especially if it is hardened by heat treating to withstand the impact without being plastically deformed over time.
A stop pin is needed that can be easily installed on a fixed structure for stopping a moving mechanism, and that will absorb the impact without transmitting the shock unattenuated to the fixed structure. Such a stop pin would be even more useful if it were of a material that is soft and xe2x80x9cdeadxe2x80x9d on initial impact, and then increases in yield strength is cold work is impressed. This ideal stop pin would also be threadable, and the threads formed in the threading operation would be stronger than the material into which the stop pin is threaded so a failure, if there were one, would not be caused by failure in the pin. Finally, such a pin would be virtually chemically inert and have fatigue properties better than most known materials.
Anchors for attachment of structures to masonry substrates, such as concrete, brick and stone, are used in many applications. The most common and convenient forms of masonry anchor require only that a straight bore be drilled into the masonry and the anchor be inserted into the bore and tightened in place to grip the sidewalls of the bore.
Such anchors are replete with problems. One problem is that they are usually designed so that they exert an axial force on the anchor while it is being tightened, which prevents the gripping elements from getting a good grip on the bore side walls before the axial force pulls them off. Another problem is the limited radial range of the gripper elements. If the bore is drilled slightly oversized, as occurs often with masonry drills because of chatter of a slightly dull bit, the anchor may not expand far enough radially to exert sufficient pressure against the side walls of the bore with its gripper elements to grip the bore securely.
The gripper elements on masonry anchors have in the past presented insuperable trade-off problems to designers of such devices because the gripper elements must be soft enough to conform to the surface topography of the side walls of the bore when pressed there against by the tightening mechanism, yet be strong enough to resist the shear forces which the anchor experiences in operation. In addition, the anchor bolt and gripper elements must be strong enough to carry the axial load but also be immune to the corrosive effects of chemicals often found in the kinds of environments, such as mines, in which they are used.
Accordingly, it is an object of this invention to provide an improved attachment and method of attachment of a shape memory effect alloy element such as Nitinol to another element for load transfer. Another object of the invention is to provide an improved method for forming threads in shape memory effect alloy, particularly Nitinol. Still another object of this invention is to provide an improved actuator element having threaded attachments for connecting between a fixed member and the movable member. A still further object of the invention is to provide a method of making a Nitinol actuator that is threaded at its ends for attachment between a fixed structure and the movable structure that the actuator is to move. Yet another still further object is to provide a method of actuating a device, and in particular, and method of actuating a blind connector and a pin puller. A yet further object of this invention is to provide an improved self-locking, self-sealing, vibration absorbing, strain indicating threaded fastener, and a method for reliably, accurately and repeatably indicating tensile preload on an installed fastener. Another yet further object of the invention is to provide a stop pin that absorbs impact without significant plastic deformation, and is stronger than other conventional stop pin materials, yet is not brittle or subject to fatigue problems. Still another further object of this invention is to provide an improved self-locking, vibration absorbing threaded equipment mount. A further object of the invention is to provide an improved masonry anchor that grips the side walls of a bore without exerting axial forces before the grip is secure, conforms intimately to the minute surface topography of the sidewalls yet resists shear forces of great magnitude, and resists corrosive environments for many years.
These and other objects of the invention are attained in an element of shape memory effect alloy such as Nitinol, wherein the element is threaded by first heating it to a temperature at which its yield strength is below the yield strength of threading tools used to form the threads, and then applying a threading tool to form the threads. I have discovered that Nitinol""s unique property of increasing yield strength as cold work is applied ceases to exist above a temperature of about 800xc2x0 C. but that the strength of the material at this temperature, fortuitously, is sufficient to resist the torque applied by a threading die being screwed onto a Nitinol blank even though the Nitinol is malleable enough to permit the Nitinol to flow into and fill the space between adjacent teeth of the threading tool when they are forced into the material. Curiously, at this temperature the Nitinol is not actually cut by the cutting threads of the tap, die or other threading tool, but instead, the material flows around the cutting threads to form threads in the Nitinol. The formed threads exhibit astonishing strength which I believe to be due to a combination of characteristics of the material in this structure: 1) the material increases in yield strength as it is subjected to cold work, 2) the material is capable of great elongation, as much as 60%, before it yields, so the load exerted on a threaded member can be shared among all the threads equally instead of just a few at a time, and 3) the metallurgical qualities of the intact grain structure of the formed threads are superior to the metallurgical qualities of cut threads. This forming technique necessitates the use of slightly undersized rod, or slightly oversized holes when using conventional dies and taps, compared to the size of rods or holes used when cutting threads, since no chips are removed, but rather the metal flows into spaces between the threads of the xe2x80x9ccuttingxe2x80x9d or forming tool. The rod or hole size when using the thread forming method of this invention is about the same as the xe2x80x9cpitch diameterxe2x80x9d of the formed threads.
The characteristics of the threaded Nitinol member provide unique capabilities to various devices, illustrative ones of which are disclosed herein. The combination of shape memory effect and a threaded attachment provides, for the first time, the ability to conveniently and economically attach a Nitinol actuator with whatever strength is desired between the fixed structure and movable device. The combination in a threaded Nitinol article of low initial strength with increasing strength when subjected to cold work, plus a large elongation capability prior to failure produces a unique threaded fastener having a self-locking and strain indicating feature, an ability to share the load over all the threads of the fastener, and an ultimate yield strength exceeding that of any other known fastener. The combination in a threaded device of damping characteristics and a strength that increases with cold work, plus the ability to elongate and share the load among all the threads and among all the other fasteners supporting the load provides unique capabilities in threaded load transferring attachments made in accordance with the invention, including threaded fasteners, stop pins, masonry anchors, and mounts for machinery and equipment, offering a hitherto unavailable combination of strength, weight reduction, vibration and shock absorption, corrosion resistance, and resistance to fatigue.